1. Field Of The Invention
The present invention relates to a polystyrene resin composition having improved stiffness. More particularly, the present invention is concerned with a polystyrene resin composition comprising a rubber-reinforced polystyrene resin, a fibrous potassium titanate and a higher fatty acid and/or its metal salt in specific proportions, which composition has not only excellent appearance and impact strength but also extremely improved stiffness. The polystyrene resin composition may include a specific amount of a flame retardant in combination with a specific amount of a carbon fiber so that the polystyrene resin composition is remarkably improved in flame retardance without sacrificing the appearance, stiffness, impact strength and melt flowability of the composition. The polystyrene resin composition can be advantageously used as a material for manufacturing parts and housings of appliances, cabinets of large televisions, and the like.
2. Discussion Of Related Art
Polystyrene resins are used in a wide variety of fields. For example, they are used as materials for preparing parts and housings of appliances, and various vessels and articles for daily life, because the polystyrene resins have both good physical properties and good moldability. However, when polystyrene resins are used as materials for large parts or large articles, e.g., cabinets for large televisions and frames for air conditioners, there are problems arising from insufficient stiffness of the polystyrene resins.
It is well known in the art that the stiffness of a polystyrene resin can be improved by incorporating a glass fiber thereinto. However, incorporation of a glass fiber causes the polystyrene resin to have poor appearance and impact strength. Accordingly, various attempts have been made to improve the stiffness of a polystyrene resin without causing other problems. For example, it was attempted to incorporate a fibrous potassium titanate into a polystyrene resin such as a rubber -reinforced polystyrene resin, which is a typical example of a high impact polystyrene (HIPS), and a rubber-reinforced styrene-acrylonitrile copolymer, e.g., an acrylonitrile-butadiene-styrene (ABS) resin [see Kogyo Zairyo (Industrial Materials), vol. 28, No. 8, pages 85-97 (1980); Kogyo Zairyo (Industrial Materials), vol. 29, No. 5, pages 44-49 (1981); Osaka-furitsu Kogyo Gijutsu Kenkyusho Hokoku (Report of Osaka Prefectural Technology Research Laboratories, Japan), No. 77, pages 49-54 (1980); Polymer Engineering And Science, vol. 13, No. 6, pages 409-414 (1973); and Japanese Patent Application Laid -Open Specification No. 58-213032]. It has been reported that from the viewpoint of the smoothness and appearance of the ultimate molded article, incorporation of a fibrous potassium titanate into a polystyrene resin is preferred to incorporation of a glass fiber into a polystyrene resin. However, when a fibrous potassium titanate is incorporated into a polystyrene resin, although the stiffness, in terms of tensile strength and flexural modulus of elasticity, of the resin is remarkably improved, the impact strength of the resin is disadvantageously lowered. Therefore, the resultant polystyrene resin composition cannot be used as a material for preparing large molded articles which are required to have particularly high impact strength.
Moreover, it is often required that the polystyrene resins have not only excellent stiffness and impact strength but also improved flame retardance. In line with these requirements, various attempts have been made in the art. For example, it was attempted to impart high flame retardance to a polystyrene resin by incorporation of a flame retardant, such as an organ halogen compound and, at the same time, impart non-dripping properties to the resin by incorporation of an inorganic filler, such as asbestos (see, for example Japanese Patent Application Publication Specification No. 58-37333). In the specification, however, it is described that when the inorganic filler is incorporated into a polystyrene resin in an amount of more than 10 parts by weight per 100 parts by weight of the polystyrene resin, the flame retardance of the resultant polystyrene resin composition become unfavorably low, that is, the resultant polystyrene resin composition shows a long glowing (combustion with no flaming) duration when subjected to a flammability test in accordance with the test for flammability of plastic materials UL-94 (published by UNDERWRITERS LABORATORIES INC., U.S.A.). This means that in the above-mentioned polystyrene resin composition, the amount of an inorganic filler must be limited to a low level to ensure desirable flame retardance. With a low level of the amount of the inorganic filler, however, the stiffness improvement by incorporation of the inorganic filler is not sufficient. Therefore, the above attempt does not give a solution to the goal of providing a polystyrene resin composition having both excellent flame retardance and excellent stiffness.
As apparent from the above, a polystyrene resin composition having not only high impact strength and good appearance but also high stiffness has not yet been realized in the art. Further, a polystyrene resin composition having not only excellent mechanical strength properties but also high flame retardance has also not yet been realized.